1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and more particularly to a magneto-optical recording medium in which data is recorded and erased using the increase in temperature of a recording layer due to heating by a laser beam and data is optically read out using a magneto-optical effect.
2. Description of the Related Art
Data is recorded in an magneto-optical recording medium by thermal magnetic recording. More specifically, a laser beam is irradiated onto a recording layer in the magneto-optical recording medium; as a result, the recording layer is locally heated to a Curie temperature or more. The irradiated region of the recording layer is magnetized in the direction of an external magnetic field to form a recorded magnetic domain. On the other hand, data is read out using a magneto-optical effect. That is, a weak laser beam is irradiated onto the recording layer. In this case, the power of the laser beam is so low to such a degree that data cannot be recorded and erased. Then, a polarization plane of light reflected from or transmitted through the recording layer is rotated in accordance with the recorded state (i.e., the direction of magnetization of the recorded magnetic domain) thereof. The data is read out by detecting this rotation.
As for a conventional thermal magnetic recording method, there are two methods: a magnetic field modulation recording method and a laser power modulation recording method. According to the magnetic field modulation recording method, a laser beam with a predetermined intensity is irradiated onto a recording layer to increase the temperature thereof, and the direction of an external magnetic field is modulated in accordance with a signal to be recorded. According to the laser power modulation recording method, a laser beam with its intensity modulated in accordance with a signal to be recorded is irradiated to the recording layer under the external magnetic field with a predetermined intensity. In particular, in order to increase the linear recording density in the longitudinal direction of a recording track, the magnetic field modulation recording method is excellent. The reason for this is that the length of the recorded magnetic domain is not limited to a spot size of a laser beam in the magnetic field modulation recording method.
Hereinafter, a conventionally proposed method for overwriting data by the laser power modulation recording method will be described.
FIG. 15 is a schematic cross-sectional view showing a magneto-optical recording medium. As shown in this figure, a recording layer includes a recording/readout magnetic film 151 and a supporting magnetic film 152. The recording/readout magnetic film 151 is a perpendicular magnetic anisotropy film which has a high coercivity Hc1 and a low Curie temperature Tc1. The supporting magnetic film 152 is a perpendicular magnetic anisotropy film which has a low coercivity Hc2 and a high Curie temperature Tc2. These films are exchange-coupled with each other. Data is recorded in the recording layer by thermal magnetic recording, using an initializing magnetic field (Hi) 153 and a recording magnetic field (Hb) 154 which generate magnetic fields opposite to each other, and a laser beam 155 whose intensity is modulated in accordance with a signal to be recorded (e.g., J. Saito et al., Proc. Int. Symp. on Optical Memory, 1987, J P N, J. Appl. Phys., Vol. 26, Supplement 26-4 (1987), p. 155).
The recording/readout magnetic film 151 is for recording and reading out data, and the supporting magnetic film 152 is for assisting the recording of data into the recording/readout magnetic film 151. These films are exchange-coupled with each other by a exchange-coupling force H1-2 (H2-1) therebetween. Suppose that the magnitude of magnetization of the recording/readout magnetic film 151 and that of the supporting magnetic film 152 are M1 and M2, the thicknesses thereof are t1 and t2, and energy of a domain wall therebetween, if any, is "sgr"w, the exchange-coupling force H1-2 seen from the recording/readout magnetic film 151 is represented by the following equation:
H1-2="sgr"w/2M1t1
and the exchange-coupling force H2-1 seen from the supporting magnetic film 152 is represented by the following equation:
H2-1="sgr"w/2M2t2
At room temperature, the following relationships are obtained: Hc1 greater than H1-2, Hc2 greater than H2-1, and Hc2+H2-1 less than Hi less than Hc1, and the magnetization direction of the supporting magnetic film 152 is aligned with a direction of the initializing magnetic field (Hi) 153.
Recording data in the magneto-optical recording medium with the above-mentioned structure will be described, in which a laser beam with a low-level intensity and a laser beam with a high-level intensity are used. In the case of using a laser beam at a low level, when irradiated with the laser beam, the temperature of the recording layer reaches the vicinity of the Curie temperature Tc1 of the recording/readout magnetic film 151 and the coercivity Hc1 thereof is lower than H1-2. Thus, the magnetization direction of the supporting magnetic film 152 (i.e., the direction of Hi) in the vicinity of the Curie temperature Tc1 is transferred to the recording/readout magnetic film 151 by the exchange-coupling force H1-2. In the case of using a laser beam at a high level, when being irradiated with the laser beam, the temperature of the recording layer reaches the vicinity of the Curie temperature Tc2 of the supporting magnetic film 152. Thus, the magnetization direction of the supporting magnetic film 152 is aligned with the direction of the recording magnetic field 154 (Hb). Thereafter, in the course of cooling step, the magnetization direction of the supporting magnetic film 152 is transferred to the recording/readout magnetic film 151 by the exchange-coupling force H1-2.
As described above, data can be overwritten in the magneto-optical recording medium by these two operations.
In the conventional magneto-optical recording medium, when the length of a recorded magnetic domain to be read out becomes less than the spot size of a readout light, recorded magnetic domains adjacent to the recorded magnetic domain to be read out are within the range of the readout light. Consequently, readout signals based on these adjacent recorded magnetic domains are detected together with a readout signal based on the recorded magnetic domain to be read out. Therefore, an S/N ratio is decreased due to the signal interference of the readout signals.
In view of the above problem, a magneto-optical recording medium having a super resolution effect has been proposed (M. Ohta et al., Proceeding of Magneto-optical Recording International Symposium ""91, J. Magn. Soc. J P N., Vol. 15, Supplement No. S1 (1991), p. 319). According to the super resolution effect, the spot size of readout light apparently becomes smaller. Readout of data by using this effect is called readout by magnetically induced super resolution. An exemplary structure of a magneto-optical recording medium for super resolution readout will be described with reference to FIGS. 14A and 14B.
FIG. 14A is a top plan view of the magneto-optical recording medium, and FIG. 14B is a cross-sectional view thereof. In these figures, the reference numeral 141 denotes an initializing magnetic field Hi, 142 a recording magnetic field Hr, 143 readout light, 144 a readout light spot, 145 a recorded magnetic domain, 146 a region at a temperature of Td or more, 147 a readout magnetic film made of a perpendicular magnetic anisotropy film with a low coercivity Hc1, 148 a recording magnetic film made of a perpendicular magnetic anisotropy film with a high coercivity Hc2. The readout magnetic film 147 and the recording magnetic film 148 are exchange-coupled with each other by a exchange-coupling force H1-2 (H2-1) to form a recording layer.
At room temperature, the coercivity Hc1 of the readout magnetic film 147 is set to be greater than the exchange-coupling force H1-2. In addition, at room temperature, the following relationships: Hc1+H1-2 less than Hi less than Hc2 and Hc2 greater than H2-1 are obtained. Data is recorded by thermal magnetic recording in the recording magnetic film 148 as the recorded magnetic domain 145 under the recording magnetic field 142. Since the relationships: Hc1 greater than H1-2, Hc2 greater than H2-1 and Hc1+H1-2 less than Hi less than Hc2 are obtained at room temperature, the magnetization direction of the readout magnetic film 147 is aligned with the direction of the initializing magnetic field 141, and the recorded magnetic domain 145 is not present in the readout magnetic film 147.
When the temperature of the region 146 of the readout magnetic film 147 is increased to a predetermined temperature Td or more by the irradiation of readout light during reading out data and the coercivity Hc1 becomes smaller than the exchange-coupling force H1-2, the magnetization direction of the region 146 is aligned with that of the recording magnetic film 148 by the exchange-coupling force H1-2. Therefore, the recorded magnetic domain 145 of the recording magnetic film 148 is transferred to the readout magnetic film 147. Thus, recorded data can be read out as a readout signal only from a portion at a temperature of Td or more of the readout light spot. That is, data can be read out from a recorded magnetic domain with a length less than the readout light spot without any signal interference by adjacent recorded magnetic domains.
The common structure of the above-mentioned two types of magneto-optical recording media (i.e., the magneto-optical recording medium for laser power modulation overwrite and the magneto-optical recording medium for super resolution readout) is as follows:
The recording layer is constituted by two or more magnetic films which are exchange-coupled with each other. At room temperature, the magnetization direction of one of the magnetic films is aligned in one direction (initializing operation). When the temperature goes up, the magnetization direction of the other one of the magnetic films is transferred to one of the magnetic films by the exchange-coupling force (transfer operation).
The above-mentioned magneto-optical recording media have disadvantages. That is, a strong magnetic field (i.e., 3 kOe or more) is required for the initializing magnetic field for the initializing operation, causing an enlarged player. In addition, it is difficult to select and combine the temperature dependence of the coercivity of each magnetic film and the domain wall energy therebetween, which enable satisfactory initializing and transfer operations. More specifically, the selection and combination of a composition of each magnetic film are difficult to realize. Moreover, in the case where a ferrimagnetic film having a compensation temperature of not less than room temperature is used for either one of the magnetic films, it becomes difficult to record data in a recording magnetic film.
The magneto-optical recording medium of this invention includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording magnetic film for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force; and a controlling magnetic film, provided between the recording magnetic film and the readout magnetic film, for controlling the exchange-coupling force, and wherein the controlling magnetic film has in-plane magnetic anisotropy at room temperature, thereby suppressing the exchange-coupling force between the recording magnetic film and the readout magnetic film, and when the temperature of the controlling magnetic film reaches a predetermined temperature by a readout light irradiation, the controlling magnetic film stops the suppression of the exchange-coupling force, whereby the information recorded in the recording magnetic film is magnetically transferred to the readout magnetic film.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording magnetic film for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; and a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force; wherein the readout magnetic film has in-plane magnetic anisotropy at room temperature, and when the temperature of the readout magnetic film reaches a predetermined temperature by a readout light irradiation, the readout magnetic film is a perpendicular magnetic anisotropy film.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording magnetic film for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force; and a controlling magnetic film, provided between the recording magnetic film and the readout magnetic film, for controlling the exchange-coupling force, and wherein the readout magnetic film has in-plane magnetic anisotropy at room temperature, and is a perpendicular magnetic anisotropy film when the temperature of the readout magnetic film is increased to a predetermined temperature by a readout light irradiation, wherein the controlling magnetic film has a compensation temperature which is substantially equal to the predetermined temperature and a Curie temperature which is set in the range from the predetermined temperature to a temperature lower than the highest temperature which the controlling magnetic film can reach by a readout light irradiation, whereby the information recorded in the recording magnetic film is magnetically transferred to the readout magnetic film via a region having a temperature in the range of the predetermined temperature to the Curie temperature.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means wherein the recording means includes: a recording magnetic film for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force; a controlling magnetic film, provided between the recording magnetic film and the readout magnetic film, for controlling the exchange-coupling force; and a switching magnetic film for breaking the exchange-coupling force between the recording magnetic film and the readout magnetic film at a temperature higher than a predetermined temperature, the switching magnetic film being provided between the recording magnetic film and the readout magnetic film, wherein the controlling magnetic film is a ferrimagnetic film having in-plane magnetic anisotropy at room temperature, thereby suppressing the exchange-coupling force between the recording magnetic film and the readout magnetic film at room temperature, and when the temperature of the controlling magnetic film reaches the predetermined temperature by a readout light irradiation, the controlling magnetic film stops the suppression of the exchange-coupling force, whereby the information recorded in the recording magnetic film is magnetically transferred to the readout magnetic film, wherein the switching magnetic film has a Curie temperature which is set to be a temperature lower than the highest temperature which the switching magnetic film can reach by the readout light irradiation, whereby the information recorded in the recording magnetic film is magnetically transferred to the readout magnetic film via a region having a temperature in the range of the predetermined temperature to the Curie temperature, and wherein the readout magnetic film has in-plane magnetic anisotropy at room temperature, and is a perpendicular magnetic anisotropy film when the temperature of the readout magnetic film reaches the predetermined temperature by the readout light irradiation.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording magnetic film having a Curie temperature, for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; and a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force, wherein, just under the Curie temperature of the recording magnetic film, the dominant sub-lattice magnetization type of the recording magnetic film is the same as that of the readout magnetic film.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording magnetic film having a Curie temperature, for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force; and a controlling magnetic film, provided between the recording magnetic film and the readout magnetic film, for controlling the exchange-coupling force, and wherein, just under the Curie temperature of the recording magnetic film, the dominant sub-lattice magnetization type of the recording magnetic film is the same as those of the readout magnetic film and the controlling magnetic film.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording magnetic film having a Curie temperature, for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force; a controlling magnetic film, provided between the recording magnetic film and the readout magnetic film, for controlling the exchange-coupling force; and a switching magnetic film for breaking the exchange-coupling force between the recording magnetic film and the readout magnetic film at a temperature higher than a predetermined temperature, the switching magnetic film being provided between the recording magnetic film and the readout magnetic film, the switching magnetic film being a perpendicular magnetic anisotropy film, and wherein, just under the Curie temperature of the recording magnetic film, the dominant sub-lattice magnetization type of the recording magnetic film is the same as that of the controlling magnetic film, and the information recorded in the recording magnetic film is magnetically transferred to the readout magnetic film due to the exchange-coupling force by a readout light irradiation.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording/readout magnetic film for recording the information and for optically reading out the information, the recording/readout magnetic film being formed of a perpendicular magnetic anisotropy film; a supporting magnetic film capable of being magnetically coupled with the recording/readout magnetic film by an exchange-coupling force; and a controlling magnetic film, provided between the recording/readout magnetic film and the supporting magnetic film, for controlling the exchange-coupling force, and wherein the controlling magnetic film has in-plane magnetic anisotropy at room temperature, thereby suppressing the exchange-coupling force between the recording/readout magnetic film and the supporting magnetic film, and when the temperature of the controlling magnetic film reaches a predetermined temperature by a recording light irradiation, the controlling magnetic film stops the suppression of the exchange-coupling force, whereby the magnetization direction of the supporting magnetic film is magnetically transferred to the recording/readout magnetic film.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording/readout magnetic film for recording the information and for optically reading out the information, the recording/readout magnetic film being formed of a perpendicular magnetic anisotropy film; a supporting magnetic film capable of being magnetically coupled with the recording/readout magnetic film by an exchange-coupling force, the supporting magnetic film having a Curie temperature; and a controlling magnetic film, provided between the recording/readout magnetic film and the supporting magnetic film, for controlling the exchange-coupling force, and wherein, just under the Curie temperature of the supporting magnetic film, the dominant sub-lattice magnetization type of the supporting magnetic film is the same as that of the controlling magnetic film.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording magnetic film for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force; a controlling magnetic film provided on the recording magnetic film; and a supporting magnetic film provided on the controlling magnetic film, wherein, when the temperature is increased by a recording light irradiation, a first transfer operation in which the magnetization direction is transferred from the supporting magnetic film to the recording magnetic film via the controlling magnetic film is performed, and when the temperature is increased by a readout light irradiation, a second transfer operation in which the magnetization direction is transferred from the recording magnetic film to the readout magnetic film is performed, wherein the controlling magnetic film is a film for controlling an exchange-coupling force between the recording magnetic film and the supporting magnetic film, and the controlling magnetic film is a ferrimagnetic film which has in-plane magnetic anisotropy at room temperature and has a compensation temperature which is substantially equal to a temperature at which the first transfer operation is performed, and wherein the readout magnetic film has in-plane magnetic anisotropy at room temperature and is a perpendicular magnetic anisotropy film at a temperature at which the second transfer operation is performed.
According to another aspect of the invention, a magneto-optical recording medium includes recording means for recording information and a substrate for supporting the recording means, wherein the recording means includes: a recording magnetic film for recording the information, the recording magnetic film being formed of a perpendicular magnetic anisotropy film; a readout magnetic film for optically reading out the information, the readout magnetic film being capable of being magnetically coupled with the recording magnetic film by an exchange-coupling force; a controlling magnetic film provided on the recording magnetic film; and a supporting magnetic film provided on the controlling magnetic film, the supporting magnetic film having a Curie temperature, wherein, when the temperature is increased by a recording light irradiation, a first transfer operation in which the magnetization direction is transferred from the supporting magnetic film to the recording magnetic film via the controlling magnetic film is performed, and when the temperature is increased by a readout light irradiation, a second transfer operation in which the magnetization direction is transferred from the recording magnetic film to the readout magnetic film is performed, wherein, just under the Curie temperature of the supporting magnetic film, the dominant sub-lattice magnetization type of the supporting magnetic film is the same as that of the controlling magnetic film.
Thus, the invention described herein makes possible the advantage of providing a magneto-optical recording medium in which an initializing magnetic field for an initializing operation is decreased or made unnecessary, the composition of each magnetic film for satisfactory performing initializing and transfer operations can be selected from a wide range, and reliable recording operations are performed even in the case where a recording film includes two or more magnetic films.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.